Trace gas detection has applications ranging from explosive and chemical weapons detection to semiconductor manufacturing and medical diagnostics, which utilize a large range of platform technologies. For many applications such as explosive vapor plume detection, extreme sensitivity is required due to the low concentrations and small cross sections of common explosives. Concentrations can be below 1 part per billion (ppb) and a typical optical cross section for TNT is less than 10−17 cm2 (and can be several orders of magnitude smaller, depending on the wavelength).
Absorption measurements represent one leading technique for trace gas detection. Small changes in the transmitted intensity of a probe laser beam are used to determine the presence of absorbing particles. In the photon shot noise detection limit, the single pass sensitivity is
      α    sn    =                    (                              2            ⁢            e            ⁢                                                  ⁢            B                                η            ⁢                                                  ⁢            P                          )                    1        /        2              ⁢          1      L      where the αsn is the minimum measurable absorption, B is the measurement bandwidth, η is the detector responsivity (A/W), e is the electron charge, and P is the power incident on the detector and L is the interaction length. For 1 mW of incident light and 1 second measurement times, absorption sensitivities approaching 10−9/cm are in principle attainable. Precise stabilization of the laser intensity is generally seen as required for this technique. In practice, a much worse sensitivity is usually achieved.
The sensitivity of absorption detection of trace gases can be enhanced by placing the absorbing sample in a high finesse optical cavity which allows multi-pass interaction between the probe beam and the sample. The light can interact with a particle each time it reflects off the cavity mirrors, which can be greater than 105 times (corresponding to cavity finesse, F˜105), as can be achieved with state of the art mirror technology in certain wavelength ranges. This provides a signal amplification approximately equal to the finesse, so that one obtains a shot noise limited minimum sensitivity of
      α    sn    =                    (                              2            ⁢            e            ⁢                                                  ⁢            B                                η            ⁢                                                  ⁢            P                          )                    1        /        2              ⁢                  π                  2          ⁢          F          ⁢                                          ⁢          L                    .      
A high finesse cavity is also a narrow frequency discriminator which only allows the transmission of a narrow range of frequencies. Therefore any frequency fluctuations in the probe laser are mapped onto amplitude fluctuations in the light transmitted through the cavity, which can further exacerbate the technical problem of intensity stabilization. One approach to circumventing this problem is to measure the decay curve of the intensity transmitted through the cavity when the light is extinguished. The light intensity can be fit to an exponential decay function, whose time constant τ (the cavity ring down time) is related to the cavity loss which includes absorption through the cavity, as discussed in K. Lehmann, U.S. Pat. No. 5,528,040. For a two-mirror cavity
      1          c      ⁢                          ⁢      τ        =      α    +                            2          ⁢                      (                          1              -              R                        )                          +        A                    2        ⁢        L            where c is the speed of light, R is the mirror reflectivity, and A denotes the loss due to scatter and absorption in the mirrors. The measured ring down time of the cavity containing the absorber is then compared to the empty cavity ring down time τempty to extract the additional absorption due to a trace element
            1      τ        -          1              τ        empty              =      c    ⁢                  ⁢          α      .      This measurement is, by design, independent of any light intensity fluctuations. The background trace is often taken in close temporal proximity to the signal trace, however since they are nevertheless separated in time, additional noise can be introduced in the measurement, due, for example, to slight changes in the cavity properties as well as other technical noise sources.
For narrow linewidth absorbers, multiple frequency laser beams can be transmitted simultaneously through the cavity separated in frequency by an integer multiple of the free spectral range (FSR=c/2 L). If one of the laser beams is resonant with the trace absorber and additional beams are far-off resonant, then the measured relative phase shift between the beams can be used to determine the absorber concentration (Ye 1998). Since the multiple-frequency measurements utilize a common cavity, and the frequency sidebands can be derived from a single laser source, much of the noise is common mode and cancels, and therefore this technique is often referred to as noise-immune. It has succeeded in measuring absorption sensitivities in molecular overtones approaching the photon shot nose limit.
For broadband absorbers, it is often not feasible to discriminate between comparable absorption levels at separated wavelengths, where both wavelengths are derived from the same master laser. Thus, one cannot implement conventional noise-immune frequency discriminating techniques with one frequency sideband far off-resonant from the absorber and one on resonance. Explosives represent such absorbers where, for example, the linewidth of a TNT resonance in the mid-IR region (7 microns) is ˜0.1 microns.
There is thus a need for trace gas detectors with improved sensitivity and ability to handle broadband absorbers.